This invention relates generally to pneumatic lung analogs and more particularly, is directed to a pneumatic lung analog capable of testing ventilatory devices intended for use with spontaneously breathing patients.
In general, ventilatory devices may be divided into two categories: devices intended to support apneic (non-spontaneously breathing) patients, commonly termed control ventilation; and devices intended to be used in conjunction with spontaneously breathing patients. Frequently, a single device may provide modes of operation covering both categories of use. This invention deals specifically with the testing of ventilatory devices which are intended to be used (or have such operational modes) with spontaneously breathing patients. Examples of such equipment include, demand oxygen inhalation devices, "assist" ventilators, and intermittent mandatory ventilation (IMV) devices.
In the case of a demand oxygen inhalation device, the patient provides a small continuous negative pressure signal to the device produced by his inhalation, and this signal automatically provides a flow of oxygen to the patient until he begins to exhale, at which time the pressure signal reverses from negative to positive. During inhalation, the patient must maintain a slight negative pressure within the device, and this requirement, in effect, imposes an additional breathing work load on the patient which is of concern. However, the disadvantage, if small, can be significantly outweighed by the attendant advantages of enriched oxygen breathing, and the "on demand" feature conserves the supply of oxygen.
"Assist" ventilators are designed to be triggered by the patient's initial spontaneous breathing effort. After sensing an initial negative airway pressure or flow produced by the patient, then such a device applies positive pressure to the patient for the inspiration of breathing gas. During such inspiration, no effort is required by the patient. Termination of each inspiration is automatic, and may depend on time, delivered volume, or pressure. In testing this type of device, the demand made on the patient to trigger it to an "on" state is of concern, and involves a knowledge of peak negative pressure needed, flow, response time, and added work of breathing.
Intermittent Mandatory Ventilators (IMV) are so termed because they intermittently and automatically apply pressurized gas to the patient at preset intervals. However, between such intermittent machine breaths, the patient is expected to breath through the device spontaneously. During these periods, the breathing gas is preferably conditioned as needed by the patient (for example, oxygen and humidity may be added, and the temperature controlled); also, instead of allowing the patient to exhale fully down to atmospheric pressure, a low pressure plateau of up to 30 cm. H.sub.2 O pressure above atmospheric, is frequently maintained. This is called constant positive airway pressure (CPAP) and is very useful under certain conditions in treating patients with pulmonary disease. However, in all cases for the patient on IMV, between mandatory breaths the patient must, on his own effort, draw breathing gas from the ventilator. Any internal resistance in the ventilator causes the patient's work of breathing to increase over open breathing where the patient is not connected to the ventilator. Some IMV ventilators incorporate servo mechanisms to reduce this work of breathing, but in all cases the patient must generate a small subatmospheric (or sub-CPAP) pressure in order to cause gas flow, and this always causes some additional work of breathing. This added work of breathing is of concern in the testing of IMV ventilators.
Pneumatic lung analogs for testing ventilators are known in the prior art. Such machines must be capable, through adjustment, of simulating a wide range of patient pulmonary physiology, ranging from pediatric to large, adult patients with various types and states of pulmonary disease. One such device is the training test lung disclosed in U.S. Pat. No. Re. 29,317 reissued July 26, 1977 to Mosley et al, the disclosure of which is hereby incorporated by reference.
The training test lung disclosed by Mosley et al simulates human lungs by providing a pair of expansible chambers or bellows which are secured at one end to a frame and at the other end to a pair of movable end plates. The bellows are interconnected by a Y-shaped fitting to assistant ventilation equipment for inflation. The movable end plates pivot relative to the frame as the bellows are inflated and the free end of the pivotable plate cooperates with printed indicia on a panel adjacent thereto, to provide a visual readout of the inspired tidal volume. Pulmonary compliance is simulated by adjustable springs which interconnect the pivotable plates and the stationary frame. Respiratory flow resistance is simulated by calibrated flow resistant tubes disposed in the flow path extending between the expansible bellows and the assistant ventilator.
Since control ventilators apply positive pressure to inflate the patient's lungs, when a patient is being treated by a control ventilator, the actual work of breathing is being carried out by the ventilator. This sometimes creates a problem, since with the extended periods of assisted ventilation, the patient begins to lose the ability to generate the work of breathing himself. Such patients often become quite dependent upon the ventilator and it may become difficult to wean these patients away from the ventilator. This problem led to the development of ventilators capable of carrying out intermittent mandatory ventilation. The newer IMV ventilators solve this problem by allowing a patient that is not apneic to breathe simultaneously whenever possible. Such a patient may be breathing weakly, erratically, or undependably, but there are advantages to allowing such a patient to generate the work of breathing himself whenever possible. Typically, such a ventilator provides a pathway for the patient to breathe through on his own,. However, such ventilators also periodically apply a positive airway pressure to create a mandatory breath. This type of ventilator can protect a patient who is capable of becoming apneic, and can be used to wean a patient away from assisted ventilation. When an IMV ventilator is used with a patient that is capable of accomplishing the work of breathing himself, the added work of breathing created by the ventilator, is of course, critical to the successs of the treatment. Thus, there is a need for a test device that simulates the spontaneous breathing patient and measures the added work of breathing created by an IMV ventilator. To the applicant's knowledge, there are no economically feasible and successful devices in the prior art for testing an IMV ventilator, or any other type of ventilator intended for use on spontaneously breathing patients, in this fashion.